Energy stocks had their best day in six weeks as the U.S. and Iran traded strikes near the Strait of Hormuz

The Energy sector rose 2.10% on a cap-weighted basis on Monday, 31 August — its biggest single-day move in the 31 most recent trading sessions in our data, going back to 20 July. The S&P 500 itself fell 0.28% the same day.

The trigger

AP News reported the U.S. struck Iranian sites in the Strait of Hormuz on 31 August, and oil moved immediately: Brent crude topped $90 a barrel intraday, and WTI rose more than 2%, as renewed U.S.-Iran hostilities revived supply concerns around the strait, the shipping route for roughly a fifth of the world's oil.

• AP News: "Oil prices rise and stocks fall after US hits Iranian sites in the Strait of Hormuz"
• Coinpaper: "Crude Oil Prices Surge as Brent Tops $90 on Iran-Hormuz Escalation"
• WSJ: "Oil Futures Eke Out Monthly Gains as U.S. Resumes Strikes"

This was a reversal from earlier in the same week: as recently as 26 August, oil coverage was running the other way, with WSJ and CNBC both reporting prices little changed or falling on hopes that Iran and Oman would reach a deal to keep the strait open. The 31 August strikes flipped that narrative inside a few trading sessions.

Where it showed up on our map

19 of the sector's 21 constituents closed higher. $SLB led with a 4.83% gain, followed by broad, uniform strength across the majors: $XOM rose 2.71%, $CVX rose 2.12%, $DVN rose 2.45%, $KMI rose 2.15%. Only $EQT, a natural-gas producer with less direct crude exposure, closed lower, down 0.71%.

Energy was the best-performing of the 11 sectors we track by a wide margin — the next-best, Technology, rose just 0.31% — while six sectors, including Communication Services (-1.59%) and Industrials (-1.08%), closed down more than 0.5%. That split is consistent with the AP framing of oil's jump as a broader headwind for equities generally, not just a tailwind for producers.

What we are not claiming

A 19-of-21 up day for the Energy sector is not unusual by itself in our data — sessions with 18 or more of the 21 names higher have happened roughly five times in the last 31 sessions. What was unusual was the size of the move: no session in that 31-day window produced a larger cap-weighted gain for the sector than today's 2.10%. We are not asserting a specific dollar impact from the strikes on any single company's future earnings, only describing the price and headline reaction on the day.